Lymphomas of the B lymphocyte lineage have been studied in a number of systems. They occur in high frequency in NFS mice congenic for ecotropic murine leukemia virus induction loci of AKR and C58 mice. These lymphomas can be transplanted and grown as continuous in vitro lines. They include mature B cell, pre-B cell, and what has been termed pro-B cell lymphomas. Some lymphomas of each type express the cell surface antigen Ly-1 and one lymphoma was found to produce autoantibodies to bromelain-treated mouse red blood cells. Pro-B cells appear to be the in vivo target cell for both Abelson and Cas-NS-1 transforming viruses. Continuous lines of non-transformed B cells have been established in culture. The cells are uniformly Ly-1+ and express Lambda light chains. Their growth is completely inhibited by treatment with anti-IgM antibodies.